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Levelling Introduction.pptx
- 1. How to: Some Basic Principles for Leveling Present by Saif Ali
- 2. Lecture Overview • Equipment • Introduction to Leveling • Observation, Field Notes, and Computation • Errors and their effects
- 3. Equipment
- 4. Equipment • Level Instrument • Tripod • Staff/Pole • Change plate (German: Frog/Frosch) • Pole staff bubble (bull eye) • Marker
- 5. Equipment: Level Instrument • Automated Levels ■ Easy to use (not power!) ■ Needs experience ■ Robust even in hostile environment • Digital Levels ■ Push-button technique ■ No reading errors, special staff ■ Readings are stored and analyzed digitally
- 6. Automated Levels (Compensator) Pendulum Tribrach Courtesy: Deumlich, Vermessungskunde Bull Eye
- 7. Digital Levels • Uses Barcode staffs • Internal storage of data ■ Download to the computer ■ Automated height computation + adjustment ■ No feeling for quality anymore ■ You frequently need power plugs
- 8. Equipment • Level Instrument • Tripod • Staff/Pole • Change plate (German: Frog/Frosch) • Pole staff bubble (bull eye) • Marker
- 9. Equipment: Tripod • Wooden design or aluminum ■ From “easy to sit” to “ops, this is high”
- 10. Equipment • Level Instrument • Tripod • Staff/Pole • Change plate (German: Frog/Frosch) • Pole staff bubble (bull eye) • Marker
- 11. Equipment: Staff/Pole • Wood, aluminum • INVAR type for high precision leveling Conventional (“E”-type) Barcode for Digital Levels
- 12. Equipment • Level Instrument • Tripod • Staff/Pole • Change plate (German: Frog/Frosch) • Pole staff bubble (bull eye) • Marker
- 13. Equipment: Change Plate • For long survey lines • Allows change of instruments ■ Best is a metal change plate ■ Screws e.g. at fences ■ Sharp stones or nails • Beware of dark colors It’s not the Indonesian- German Dictionary, It’s the nail!
- 14. Equipment • Level Instrument • Tripod • Staff/Pole • Change plate (German: Frog/Frosch) • Pole staff bubble (bull eye) • Marker
- 15. Equipment: Bubble • Keep the pole upright ■ Any tilt will disturb your readings
- 16. Equipment • Level Instrument • Tripod • Staff/Pole • Change plate (German: Frog/Frosch) • Pole staff bubble (bull eye) • Marker
- 17. Survey Markers • Gives you a fixed point ■ Should be of good quality ■ Should be long-term ■ Preferable in bedrock, settled buildings, or bridges ■ Do not use fences or walls
- 19. Some Basic Definitions • Level surface (e.g. the geoid) ■ A water surface with no motion ■ Gravity gradient is the normal to the level surface ■ The Instrument’s Bubble is in the normal (!) • Horizontal surface ■ At the instruments axis, the horizontal surface is tangent to the level surface ■ Over short distances (<100 m) the horizontal surface and the level surface will coincide ■ For long leveling lines the effects of the gravity field must be considered
- 20. Leveling rods Line of sight Back sight Fore sight Gravity Gradient Basic Principle of Leveling • Measures height differences between points ■ Along a line ■ Several points from one occupation bs fs Δh = bs - fs
- 21. Definitions • Back sight (BS) ■ The first reading from a new instrument stand point (i.e. take the height to the instrument) • Fore sight (FS) ■ The last reading from the current instrument station (i.e. give the height to a benchmark) • Intermediate sight (IS) ■ Any sighting that is not a back sight or fore sight
- 22. Reading a Staff 1422 • Read the [m], [dm] & [cm] • Estimate the [mm] • Check yourself for frequent used numbers (2/3) or (7/8)
- 23. Basic Rules for Leveling • Always start and finish a leveling run on a Benchmark (BM or TGBM) and close the loops • Keep fore sight and back sight distances as equal as possible • Keep lines of sight short (normally < 50m) • Never read below 0.5m on a staff (refraction) • Use stable, well defined change points • Beware of shadowing effects and crossing waters
- 24. Observation, Field Notes, and Computation
- 25. How to: A sample loop S2 S1 New Benchmark NB1 Tidal Hut TH New Benchmark NB2
- 26. How To: Field Notes Back Inter Fore Point TH NB1 NB2 NB1 TH 1327 3982 S2 S1 New Benchmark NB1 Tidal Hut TH New Benchmark NB2 2365 0986 2347 3724 3753 1101
- 28. Compute levels Back Inter Fore dh H Comment TH BM1 BM2 BM1 TH IST SOLL 1327 3982 0986 3724 2365 2347 3753 1101 1379 2652 9792 9793 -0001 +1 100 000 100 000 0000 0001 (SOLL – IST) ´7345 ´8624 0 97 345 98 724 97 348 ? 0
- 29. Loop misclosure • Misclosure Error ■ The difference of the measured height difference (ΔHmeas) to the known height (closed loops = 0, known benchmarks = height difference) Misclosure = ΔHSOLL – ΔHIST • Point errors at double observed points
- 30. Achievable Accuracy • Instrument dependent ■ Roughly from the instrument ⬥NI002 = 0,2mm/km (doubled line) ⬥NI025 = 2.5mm/km (doubled line) • Survey line length dependent ■ ms = m1km √s, s in km ■ mH = (m1km/2) √s, s in km #(middle of the line)
- 31. An acceptable misclose? • Small misclosures in closed level loops are expected because of the accumulation of random errors and can be adjusted • If the misclosure is large, the loop (or part of it) must be repeated • Misclosures can also result from errors in published BM levels and from BM instability
- 32. Testing the misclose • The amount of misclosure acceptable using a specific instrument and survey line length • For our example, a second order leveling standard is adopted*… misclosure ≤ 2,5√s mm • where s is the length of the line in km *Dependent on your contry’s rules and the instrument used
- 33. Our example • The misclosure is +1 mm • The length of the loop is 0.4 km • Acceptable error is 2.5√(0.4) = ±1.6 mm • The misclosure of +1 mm is within the limit • Mean error for NB1 = 2.5/2* √(0.4)
- 34. Errors and their effects (many, but only a few addressed)
- 35. Errors in leveling, e.g. • Collimation, Parallax • Change point / staff instability • Instrument or Benchmark instability • Refraction • Uncalibrated staff or levels • Reading, booking, or computation errors • Fore- and backsight distances different
- 36. Systematic and Random Errors • Earth curvature • Refraction • Collimation errors
- 37. Effect of Earth Curvature 80 0,8 0,2 0,03 0,008 Effect (Δh) in mm 1000 100 50 20 10 Distance (s) in m www.fh-oow.de/institute/ima/personen/weber/VK_12/VL_VK1/geo_niv_6.htm Horizontal Level (r +Δh)2 = r2 + s2 => Δh ≈ s2/(2r)
- 38. Refraction www.fh-oow.de/institute/ima/personen/weber/VK_12/VL_VK1/geo_niv_6.htm Mean Gradient: 0,2 °C / m
- 39. Collimation error • Occurs when the line of sight (as defined by the lens axis and cross-hairs) is not horizontal • Leads to an incorrect staff reading horizontal line error
- 40. Stand point 1 Stand point 2 ´ ´ ´ ´ Instrument test: Nähbauer a′1 = a1+e b′1 = b1+2e Δh = a1−b1 Δh′1 = a′1−b′1 = a1−b1−e = Δh−e With Δh′1+e = Δh′2−e Δh′2−Δh′1 2 a′2 = a2+2e b′2 = b2+e Δh = a2−b2 Δh′2 = a′2−b′2 = a2+e−b2 = Δh+e Δh =Δh′2−e e =
- 41. Summary
- 42. Procedure of leveling 1. The instrument must be check before use! (see lecture) 2. The instrument and level must be stable settled-up 3. The bubble tube must be leveled before the reading • Beware of sun exposure (will wander) • Ensure the instruments pendulum is in-limit 4. The instrument must be set up in the middle between two staffs • Prevents curvature effects • If impossible, use the same distances, but opposite for the next readings 5. You must not use the parallax screw between the backsight and foresight readings
- 43. 6. Readings must be taken 30-50 cm above the ground ● Surface refractions ● Beware also of temperature gradients (inside/outside buildings) !!!! 7. Staff should be set up vertically 8. A change plate should be used 9. Leveling must be done in two opposite directions but the same line (beware of gravity gradients) 10. Staff should be calibrated, especially if INVAR 11. Be careful when crossing rivers (large water surfaces) ● Use “same-time” (mutual) observations ● Repeat it during different times of the day Procedure of Leveling
- 44. An Unhappy Surveyor … having a 2 centimeter difference